In welding automobile bodies and automobile components, resistance spot welding, typically direct spot welding, has been conventionally used. Recently, for example, a series spot welding method and an indirect spot welding method have been used.
Characteristics of these three types of spot welding will be described with reference to FIG. 1.
All the three types of spot welding are the same in that at least two overlapping steel sheets are joined together by welding.
FIG. 1(a) illustrates a direct spot welding method. As illustrated, the direct spot welding is a method in which a point-like welded portion 5 is obtained, through use of resistance heat generated between two overlapping metal sheets 1 and 2, by allowing current to flow through the metal sheets 1 and 2 while applying pressure to a pair of electrodes 3 and 4 sandwiching the respective metal sheets 1 and 2 from above and below. The electrode 3 is provided with a force controlling system 6 and a current controller 8, and the electrode 4 is provided with a force controlling system 7 and the current controller 8. This mechanism makes it possible to control welding force and current.
A series spot welding method illustrated in FIG. 1(b) is a method in which point-like welded portions 15-1 and 15-2 are obtained by allowing current to flow through two overlapping metal sheets 11 and 12 while applying pressure to a pair of electrodes 13 and 14 at separate locations from the same side (in the same direction).
An indirect spot welding method illustrated in FIG. 1(c) is a method in which a point-like welded portion 25 is formed between two overlapping metal sheets 21 and 22 by holding an electrode 23 against the metal sheet 21 while applying pressure to the electrode 23, attaching a feeding point 24 to the other metal sheet 22 at a location remote from the electrode 23, and allowing current to flow between the electrode 23 and the feeding point 24.
Of the three welding methods described above, the direct spot welding method is used when there is enough room and it is possible to create an opening which allows the metal sheets to be sandwiched from above and below.
In actual welding, however, it is often difficult to make enough room and to sandwich the metal sheets from above and below in a closed cross-sectional structure. The series spot welding method or the indirect spot welding method is used in such cases.
However, when the series spot welding method or the indirect spot welding method is used in applications such as those described above, the overlapping metal sheets are pressed by an electrode from only one side, with the other side of the metal sheets being unsupported and hollow. Therefore, unlike in the case of the direct spot welding method where the metal sheets are sandwiched by electrodes on both sides, it is not possible to apply high welding force locally to a point directly below the electrode. Moreover, since the electrode sinks into the metal sheet in contact therewith during application of current, conditions of contact between the electrode and the metal sheet and between the metal sheets change. This results in an unstable current path between the overlapping metal sheets, and makes it difficult to form a fused junction.
As a solution to the problems described above, Patent Literature (PTL) 1 discloses a series spot welding method in which “to form nuggets at points of contact between overlapping metal sheets, steady-state current is applied after forming electrode nuggets by application of large current at an early stage of welding process”. PTL 2 discloses another series spot welding method in which “sufficient welding strength can be obtained, without use of back electrodes, by forming seats one step higher than other portions, and pressing electrodes into contact with the respective seats such that the seats are compressed”.
As for indirect spot welding, PTL 3 discloses a welding method which is also applicable to series spot welding. Specifically, PTL 3 discloses a welding method in which “during application of current in series spot welding or indirect spot welding, a time period during which current is kept high and a time period during which current is kept low are repeated alternately”. PTL 3 also discloses a welding method in which “during alternate repetition of a time period during which current is kept high and a time period during which current is kept low, the level of current in time periods during which current is kept high is gradually increased”.